If I setup my router (2850vn) to handle PSTN calls and two VoIP lines, is there any way of identifying which line is ringing when a call comes in? I'm using a VoIP line for business calls and it would be useful to know (from the handset) which line is ringing so I can answer it appropriately. For example, is it possible to control the CLI that's displayed on the handset?

This is not quite what you are asking - but on my vigor 2800, then on my vigor 2850 I set the ring pattern to 1, for SIP account 1, 2 for SIP account 2 etc.
You can then tell from the ring tine which line is receiving putting the call to the handset.

Sounds like a good option. But in the end, I was able to get my VoIP provider to prefix the Caller ID. So now I can see the incoming call number and know which SIP account they're calling. Seems to work well